The present invention relates to a device, system and a method for electronic true color display, and in particular, to such a device, system and method in which an expanded color space is available for display through an electronic display device such as a monitor of a computational device, for example.
The perception of color by human vision involves the impact of light of different wavelengths in the visible spectrum (400 nm-780 nm) on the human eye, and the processing of the resultant signals by the human brain. For example, in order for an individual to perceive an object as “red”, light in the range of wavelengths of about 580-780 nm must be reflected from the object onto the retina of the eye of the individual. Depending upon the spectral distribution of the light and assuming normal color vision, the individual perceives different colors from a wide range of such colors.
In addition, the individual perceives various characteristics of the color. The color itself is also termed the “hue”. In addition, saturation determines the intensity of the color, such that a color shade which is saturated is perceived as highly vivid, while a pastel version of the same color is less saturated. The combination of hue and saturation forms the chrominance of the color. As perceived by the individual, color also has brightness, which is the apparent or perceived energy of the color, such that the color “black” is actually the absence of brightness for any color.
Although color is a complex combination of physical and physiological phenomena, as previously described, color matches to viewable colors can be obtained with combinations of only three colors, usually red, green and blue. These three colors are the additive primaries. By combining different amounts of each color, a wide spectrum of colors can be produced. Unfortunately, this spectrum still falls short of the complete spectrum of colors which are visible to the human eye (see for example www.barco.com as of Sep. 28, 2000). Not every color can be expressed as a mixture of three primary colors in combination. Instead, certain colors can only be adequately represented mathematically if the value for one or more primary colors is negative. While such negative values are theoretically possible, physical devices cannot produce them.
An international standards body, CIE (Commission Internationale de l'Eclairage), has defined a special set of imaginary primaries, for which all colors can be represented by positive values. The primaries are imaginary in the sense that they are a mathematical creation, which cannot be produced by a physical device. Nevertheless, the system is very useful for the presentation of color, as is described below.
The system is defined by the color matching functions, X(λ),Y(λ) and Z(λ), which define the response of the primaries to a monochromatic exciatation of wavelength, λ. Furthermore, Y(λ) is chosen to be identical to the brightness sensitivity of the color sensors in the human eye. Using these primaries, each color can be represented by three positive values XYZ, where Y is proportional to the brightness of the excitation. From the XYZ values a normalized set xyz is created, by dividing each of the values by X+Y+Z. In the new set x+y+z=1, thereby enabling the third value to be derived from these values, such that a color is represented by a set of x and y values on a chromaticity diagram as shown in Background Art FIG. 1. Information which is lost in the process of normalization is the brightness of the color, but all chromatic information is kept.
The chromaticity diagram in FIG. 1 describes a closed area in a shape of horseshoe in the xy space. The points on the border of the horseshoe (shown as line 10), known as the spectrum locus, are the xy values corresponding to monochromatic excitations in the range from 400 nm to 780 nm as marked. A straight line 12, closing the horseshoe from below, between the extreme monochromatic excitation at the long and short wavelengths, is named the purple line. The white point, which is the point at which the human eye perceives the color “white”, is lying inside the closed area. All colors discernible by human eye are inside this closed area, which is called the color gamut of the eye. If an excitation is monochromatic, it is placed on the horseshoe border. If it is spectrally wide, thereby containing light of a plurality of spectra, its coordinates lie inside the gamut.
The electronic reproduction of color, for example by an electronic display device such as a computer monitor, is currently performed by using three primaries: red, green and blue. These systems cannot display the full range of colors which are available to the human eye. The reason for the inability of such devices to display the full range of colors perceived by the human eye is that some colors are presented by negative values of one or more of the primaries, which cannot be realized by a physical light source. Certain background art devices and systems use a fourth “color”, which is actually light passed through a neutral filter, or “white light”, and which is used for controlling brightness of the displayed color, as described for example with regard to U.S. Pat. No. 5,233,385. However, the use of the neutral filter does not affect the ultimate spectrum of colors which can be displayed.
Electronic display devices which operate according to the three-primary red, green, blue system include such devices as computer monitors, televisions, computational presentation devices, electronic outdoor color displays and other such devices. The mechanism for color display may use various devices, such as Cathode Ray Tubes (CRT), Liquid Crystal Displays (LCD), plasma display devices, Light Emitting Diodes (LED) and three-color projection devices for presentations and display of video data on a large screen, for example.
As an example of the operation of such a device, CRT displays contain pixels with three different phosphors, emitting red, green and blue light upon excitation. In currently available displays, the video signal sent to the display specifies the three RGB color coordinates (or some functions of these coordinates) for each of the pixels. Each coordinate represents the strength of excitation of the relevant phosphor. An individual viewing the display integrates the light coming from neighboring colored pixels to get a sensation of the required color. The process of integration is automatically performed, without individual awareness of the process, and occurs through a combination of the physiological activity of the eye itself and of processing of signals from the eye by the brain.
The red, green and blue emissions of the phosphors define three points in the xy plane. The points marked 14, 16 and 18 in FIG. 2 represent red, green and blue phosphors respectively of a typical phosphor set used for televisions and related devices. As can be seen in FIG. 2, these points 14, 16 and 18 lie inside the spectral gamut of the eye, which is the range of spectral values for light visible to the human eye. Many colors can be created using these primaries. However, not all colors can be created, as previously described, since only positive values of RGB are possible. These positive combinations represent colors which are inside a triangle 20, created by the three primaries, as can be easily seen from FIG. 2. However, a significant portion of the gamut of the eye lies outside triangle 20, and therefore cannot be displayed by using the three phosphors system.
Part of this problem could be alleviated by using lasers or other spectrally narrow light, since the emission of the phosphors is spectrally wide, thereby causing the triangle of values lying within the gamut of produced colors to be even smaller. A similar problem is found with LCD display devices which operate with “white” light passed through color filters, and which must also have a wide spectrum for the filters in order for enough light to pass through the filter. However, the problem of the restricted gamut for display of colors cannot be solved by using monochromatic light sources, such as lasers; although the triangle created is much larger, large parts of the gamut of the human eye still cannot be displayed with only three primary colors, regardless of the type of light source.
A more useful solution would enable a wider range of colors to be displayed by the electronic display device, for example by a television or a computer monitor. Such a solution would be efficient and would be suitable for both large electronic display devices and more small, portable devices. Attempts to define such a solution can be found, for example, in PCT Application Nos. WO 97/42770 and WO 95/10160, which both describe methods for processing image data for display with four or more primary colors. However, neither Application teaches or suggests a device which is capable of such a display of four or more primary colors.
U.S. Pat. Nos. 4,800,375 and 6,097,367 both describe attempts to provide such devices. However, neither disclosed device is a suitable solution to this problem, as both devices have significant disadvantages. For example, U.S. Pat. No. 4,800,375 describes a flat, backlighted screen, in which the light source and controller form a single unit. However, since each pixel has a different color, increasing the number of primary colors both increases the cost of production, since additional light source/controller units must be added for each color, and also decreases the resolution of the screen. Similar problems are also found with the disclosed device of U.S. Pat. No. 6,097,367, which is based on LED (light emitting diodes). Thus, these disclosed background art devices clearly suffer from significant drawbacks, particularly with regard to the decreased resolution of the displayed image as the number of primary colors which form the image is increased.
Therefore, there is an unmet need for, and it would be highly useful to have, a device, system and a method for providing an expanded color spectrum for the electronic display and reproduction of color, which would operate efficiently and which would be suitable for display devices of different sizes, and which would not result in decreased resolution of the displayed image as the number of primary colors is increased.